A normal cathode ray tube (CRT) such as is used in a computer terminal is composed of a transparent screen with a phosphor layer formed on its inner surface. The phosphor layer is stimulated by one or more electron beams so that the layer emits light energy. This light energy is the visual representation viewed by an observer of the screen.
The screen and tube are typically not shielded against the radiation of EMI/RFI energy generated within the CRT. The ingress and egress of EMI/RFI energy must be controlled for health and security reasons. For example, recent studies have suggested that the EMI/RFI energy emanating from CRT devices may affect the health of its users. Additionally, it is known that EMI/RFI emissions from CRT devices can affect the operation of adjacent unshielded devices and may also be detected and read by highly sensitive detectors.
One approach to eliminating the movement of EMI/RFI energy through the CRT screen has been to use a transparent shield which contains an electrically conductive mesh or grid. Mounting such mesh is difficult and time consuming as the mesh tends to tear easily. The shielded structure suffers from reduced visual opacity in that the mesh interferes with one's viewing of the screen. More importantly, the use of the conductive mesh causes the formation of moire patterns which interfere with the viewing of the screen.
Moire patterns are caused by the overlaying of two patterns which are similar in spatial frequency and distribution. In CRT devices, they are caused by the overlaying of two similar grid patterns. The first grid is the wire mesh used in the EMI/RFI shielding device. The second grid is formed by the CRT scan lines and pixel delineation. The frequency and distribution of the second grid is variable as each CRT screen has different spatial distributions. The problem is more severe when using a high resolution screen, as the spatial definition of the pixels is closer to that of the wire screen, thus generating more moire patterns.
One method for avoiding the generation of moire patterns is to eliminate the wire mesh, while maintaining the EMI/RFI shielding. Various coated shields or screens have been offered and while the shields do reduce or eliminate the moire patterns, they also have major disadvantages. The main problem with such coatings is that the visual opacity of the CRT screen is significantly reduced (generally up to 50% less than on an unshielded screen). This requires the operator to increase the screen's luminescence to a higher level which reduces the useful life of the screen. Additionally, the coatings are generally less conductive than the wire mesh and therefore do not provide the same effective level of EMI/RFI shielding.
The present invention overcomes the difficulties encountered with the present devices. The shield of the present invention provides excellent EMI/RFI shielding and visual opacity without generating moire patterns.